Masked gunmen have abducted a leading Syrian human rights lawyer and three other prominent activists in a militant-held Damascus suburb in a new sign that al-Qaida-linked militants are trying to silence any opponent.

Razan Zaytouni, one of the most outspoken critics of foreign-backed militants, was seized along with her husband and two other colleagues from her office in Douma, northeast of Damascus.

No group claimed responsibility for the kidnapping, but Zaytouni herself had publicly blamed al-Qaeda-linked terrorists for kidnapping activists, and colleagues said she had received indirect threats from extremists in recent days.

Terrorists fighting in Syria have so far abducted hundreds of Syrian civilians and foreign journalists, and in several cases killed them without any justification.

On Tuesday, families of two Spanish journalists, who were kidnapped in Syria three months ago, have publicly appealed for their release.

Syria has been gripped with a deadly insurgency since 2011 when pro-reform protests turned to an all-out war by infiltration of militants and terrorist groups from across the Middle East, North Africa and Europe. More than 100,000 people have been killed in the violence, according to the UN.